1. Field of the Invention
The present invention relates to a transmission device and a home server system that select all or a part of input streams and transmit the selected streams as new streams, and more specifically to a transmission device and a home server system capable of suppressing the generation of a PCR jitter by a simple circuit component when a digital broadcast stream is received and redistributed.
2. Description of the Related Art
Recently, a digital broadcast stream, for example, there has been a receiving device widespread for recording and regenerating a digital broadcast program without degrading the quality of an image by recording as is a transport stream (hereinafter referred to as TS) of, for example, MPEG2 standards. There is a method of using the receiving device as a home server to receive a program over a network by redistributing the TS to a digital television receiver (hereinafter referred to as digital TV) connected to the network.
The TS is configured by plural packets (hereinafter referred to as a TS packet). When the packets are recorded, only necessary TS packets are normally selected from among received TS packets and recorded. Thus, selecting only necessary TS packets is hereinafter referred to as “partialization”, and the selected TS packet group is hereinafter referred to as a “partial TS” etc. The TS before partialization can also be referred to as a “full TS” for convenience.
The TS includes reference time axis information called a PCR (program clock reference), and synchronous reproduction of video and audio can be realized by a receiving device such as a digital TV by decoding compressed data multiplexed on the TS based on the time axis. The PCR is to be multiplexed to the TS packet at least once in 100 ms in accordance with the MPEG2 standards on the transmitting side, but it is not always necessary to multiplex the TS packets at equal time intervals. Therefore, the PCR is transmitted as discrete time information from the transmitting side, and an STC (system target clock) as serial time axis information is reconstructed based on the PCR on the receiving side.
In accordance with the MPEG2 standards, the receiving device is obliged to finely adjust the operation frequency of the STC (in accordance with the MPEG2 standards, an adjustment within at least 27 MHz±810 Hz is required) such that the difference value between the time indicated by the n-th PCR (hereinafter referred to as PCR(n)) and the time indicated by the (n+1)th PCR (hereinafter referred to as PCR(n+1)) can be equal to the difference value between the arrival time of the PCR(n) and the arrival time of the PCR(n+1), thereby eliminating an error between the PCR(n) and the time indicated by the STC (hereinafter referred to as an STC(n)) when the PCR(n) arrives for decoding.
Thus, an error in the time axis direction is called a “jitter”, and the time axis reconstructed by the PCR is called a “PCR jitter”. In the MPEG2 standards, the tolerance of the PCR jitter is ±500 ns, and the receiving device such as a digital TV etc. is designed based on the accuracy in error. Therefore, when the partial TS is redistributed to another digital TV over a network (IEEE1394, LAN, etc.), it should be considered not to exceed the tolerance of the error.
The partial TS is considered below
As shown in FIGS. 47A to 47D, when a partial TS (refer to FIG. 47B) as a result of selecting the TS packet “packet Ax” (x=0, 1, 2, . . . ) from the full TS (refer to FIG. 47A) is recorded, the partial TS is recorded on a recording medium (DVD (digital versatile disc), BD (Blu-ray disc), a hard disk, etc.) in a state in which the TS packet portion not selected in the full TS is filled (refer to FIG. 47C).
The resultant data is simply reproduced as follows. That is, as shown in FIG. 47D, the relative intervals of the packets A2 and A3 from the recording start TS packet “packet A0” are different from those during recording (when a full TS is input shown in FIG. 47A).
Therefore, as shown in FIG. 48A, if a PCR is multiplexed on the packets A0 and A3 when a full TS is input, the relative interval of the packet A3 from the packet A0 is different from the interval during recording (when a full TS is input as shown in FIG. 48A) in the partial TS during reproduction as shown in FIG. 48B. As a result, as shown in FIG. 48C, the STC reconstructed when a partial TS is reproduced (STC during reproduction) is largely different from the STC reconstructed during recording (STC during recording). That is, the difference between the arrival time of the PCR(n) and the arrival time of the PCR(n+1) is different between the value during recording (t(n+1)−t(n)) and the value during reproduction (t′(n+1)−t(n)), and the PCR jitter violates the value ±500 ns determined in accordance with the MPEG2 standards. In FIG. 48C, the horizontal axis indicates the arrival time of the PCR, and the vertical axis indicates the time (PCR value).
When the MPEG2 standards are violated, the receiving device cannot correctly reconstruct the operation frequency of the STC, thereby causing a defect such as a color error in a video signal, etc. TS packets other than the packets A0 and A3 are reconstructed during reproduction at a transmission rate higher than the input rate during recording, and the STD (system target decoder) buffer of the receiving device overflows, thus causing a problem in which correct reproduction cannot be performed.
Therefore, a method of suppressing the PCR jitter and the overflow during reproduction is proposed and used as recording after assigning the arrival time as a TIME STAMP to each TS packet on the receiving side (for example, the stream object (SOB) format etc. in accordance with the HD DVD standards).
However, the method of recording after assigning a TIME STAMP to a TS packet each time the packet is received on the receiving side enables the PCR jitter and the overflow to be suppressed for each packet, but a PCR jitter of a unit less than a packet cannot be suppressed.
On the other hand, as a prior art, for example, the Japanese Patent Laid-Open No. 2000-151539 discloses a data transmission device for suppressing a PCR jitter within a tolerance using a PLL.
However, there is the problem with the transmission device according to the Japanese Patent Laid-Open No. 2000-151539 that the size of PLL is large and costly parts are used.